The causal relationship is that mouth leakage tends to
dry-out the mouth (Xerostomia) by evaporating saliva while asleep. There are multiple factors which can cause Xerostomia and most of them are
far, far more significant than mouth leakage under CPAP treatment.
Amongst other things, saliva regulates acidity in the mouth, and provides protection from bactaerial infection. It is the reduction in saliva that promotes dental decay issues..... so its not a direct impact from CPAP treatment, but rather an indirect consequence arsising from a
limited and containable circumstance that may occur for some CPAP users.
You may find this citation useful...
http://jada.ada.org/cgi/content/full/136/8/1121Cheers,
Bill